McDonald's commits to "sustainable beef" -- whatever that is

Updated at 1:14 p.m. ET

McDonald's (MCD) is taking a major step to reduce its carbon footprint, vowing to eventually start buying beef from sources of verifiable "sustainable" meat. The only problem? No one knows what "sustainable" means.

Still, the fact that the fast-food giant is committing to buying "sustainable" beef is remarkable, given that phrases such as "environmentally friendly" haven't usually been paired with a Quarter Pounder or Big Mac. 

But what the pledge means for the beef industry, McDonald's own operations and consumers is still unknown. For instance, it's not clear whether the cost of its iconic burgers will rise as the chain shifts to new sources of beef.

In an emailed response to CBS MoneyWatch, McDonald's said it's too early to predict if or how beef prices will be impacted. 

Consumers will "vote with their feet more and more" on the issue of sustainability, Bob Langert, vice president of global sustainability at McDonald's, said in a statement supplied to CBS MoneyWatch.

The fast-food chain itself is admitting that it doesn't exactly know what "sustainable" means. Not yet, at least. In announcing the decision on Tuesday, the company noted, "This sounds simple, but it's actually a big challenge because there hasn't been a universal definition of sustainable beef."

To that end, McDonald's plans to work with outside organizations, including the World Wildlife Fund and Cargill, to draft principles and best practices for sustainable beef. 

Still, cynics will note that the company is only vowing to start purchasing sustainable beef in 2016, without providing concrete goals, such as what percentage of its beef might come from such sources by then. Langert told GreenBiz.com in an interview that McDonald's isn't ready to commit to a specific quantity. GreenBiz.com noted that it could take the chain a decade or more to buy 100 percent of its beef from sustainable sources. 

Beef is big business for McDonald's, with some of its iconic burgers brands pulling in more than $1 billion in annual sales each, GreenBiz.com notes. That also has had an impact on the environment, with beef representing more than one-quarter of McDonald's carbon footprint, it adds. 

To some extent, the move has McDonald's trailing smaller, pricier competitors, such as Chipotle Mexican Grill (CMG), which buys "naturally raised beef." That does lead to some shortages in some areas of the country, Chipotle notes. 

While McDonald's is making a laudable commitment, there are still a lot of unanswered questions, such as whether McDonald's fans will be willing to pay more for their sustainably sourced Big Macs or be patient if their Quarter Pounders are in short supply.